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Shielded Sucker

A cheap and easy air intake improvement - with no CAI pipe in sight!

By Michael Knowling

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The air intake is one of the most cost-effective areas for improvement on pretty well any mass produced vehicle. Even the most elaborate brand-name pod filter can be bought for a just couple of hundred dollars and - so long as it's intelligently installed - you're virtually guaranteed an improvement in go-go-go. But what if you don't want to spend more than about $20 and you're not keen on spending hours under the bonnet or with your head buried inside the wheel arch?

Well, here's an approach that's extremely cheap and easy - and, no, it doesn't involve flexible hose or PVC piping!

Our Example Vehicle...

The demo car for our low-cost and low-effort intake alteration is the 2.4-litre Nissan Pintara Ti we recently whacked a new exhaust onto; you know, the one using a large OE cat and muffler.

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The 2.4 Pintara's intake arrangement is fairly typical of systems you'll find in other EFI front-wheel-drive vehicles. The beginning of the intake system comprises an intake snorkel that, about half way along its length, T's into a plastic resonant chamber. The intake snorkel assembly feeds air into the lower half of a square-edged plastic airbox, which houses the same size filter as found in VL - VS Holden Commodores. The lid of the airbox, meanwhile, incorporates a hot-wire type airflow meter featuring a bolt-on bell-mouth entry. After passing through the airflow meter, induction air then travels through a convoluted engine pipe before arriving at the throttle body.

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On-road testing revealed the majority of our Pintara's intake restriction occurred through the snorkel leading to the airbox; note that the pre-airbox plumbing has been restrictive on all of the factory induction systems we've ever tested. Of course, the big upshot of this situation is we can make decent flow gains by modifying just the hardware leading into the airbox - easy.

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You can see here why the Pintara's intake snorkel is so restrictive - its cross-sectional area chokes down dramatically and is very turbulent near the pick-up. Furthermore, the pick-up would struggle to inhale large volumes of induction air since it is situated so close to the back of the headlight and - finally - the attached resonant chamber wouldn't be helping. Overall, the factory pre-airbox intake snorkel is a bit of an airflow disaster.

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Our pressure drop testing also revealed that the 2.4-litre Pintara suffers considerable restriction through its airflow meter. A look inside the AFM tells the story - its primary flow orifice is much smaller than the rest of the factory induction pipework.

Note that, unlike some other hot-wire airflow meters, the 2.4 Pintara 'meter is not fitted with wire mesh screens - we couldn't just flick the screens for an easy improvement in airflow (like we have done previously on a Holden Commodore VL Turbo). Unfortunately, we weren't going to be able to improve flow through the airflow meter in this case; not without finding a compatible larger 'meter and calibrating its outputs, anyway...

This table shows the distribution of the Pintara's intake airflow restriction.

Intake Section Airflow Restriction (inches of water)
Intake Snorkel 6.5
Lower Section of Airbox and Air Filter 0.5
Top of Airbox, Airflow Meter and Convoluted Engine Pipe 5
Total 12

Testing with a Jaycar LCD temperature probe showed that the Pintara's standard intake arrangement leaves further scope for improvement. As we have emphasised in previous stories, the temperature of an engine's intake air is extremely important; cool intake air maximises torque and power while also reducing the chance of detonation. In the Pintara - which picks up air from behind the left headlight - we measured temperatures inside the airbox hovering around 45-degrees Celsius in urban driving with an ambient temperature of about 15-degrees Celsius; not woeful, but less than ideal nonetheless.

Going Down - Intake Restriction and Temperature...

The approach we took on our Pintara's air intake is both extremely time and cost effective - and pretty well anyone can accomplish it at home with a few basic tools.

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The first step is to rip out the restrictive section of the factory air intake - the pre-airbox snorkel. In the Pintara's case, this meant hauling out the left front guard liner and removing the intake resonant chamber. Next, a small bracket that mounts the front of the snorkel needs to be removed and a nearby fuse box should be undone and pushed aside. You'll also need to slide the radiator overflow bottle from its mount and shove it somewhere out the way.

Now it is necessary to remove the airbox in order to withdraw the intake snorkel from the vehicle.

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Loosen the clamp that secures the airflow meter to the convoluted engine pipe, release the clips retaining the airbox lid and manoeuvre the lid/AFM assembly out of the way (being careful not to damage the airflow meter wiring). The bottom section of the airbox should then be slid off its mounts and you're now free to withdraw the intake snorkel from the vehicle.

Feel free to chuck it in the bin.

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As it turns out, the 'breath hole' in the lower half of the Pintara airbox is very large - it actually has a cross-section larger than a 3-inch diameter pipe! Previous experience has taught us that a pre-airbox induction pipe diameter of 3-inch serves well on engines generating more than 150kW; on our 100kW-odd Pintara, therefore, the existing opening was more than ample. No need to cut the box for greater airflow.

Note, however - in most cases - we suspect that the factory airbox opening will be more restrictive than the 2.4 Pintara's; in this case, we suggest enlarging the airbox opening with a file.

What is the optimal airbox breath hole size?

There's a simple rule when it comes to sizing pre-airbox intake diameters; bigger is better!

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Having said that, it is not always necessary to run a 4 or 5-inch airbox opening or feed pipe. For cars with less than about 170kW we'd suggest a 3-inch breath hole is sufficient; you won't relieve any more restriction using a larger-again pipe. For applications up to around 300kW, however, we'd suggest a 4-inch diameter breath hole is sufficient - so long as you can fit such a monster opening into your airbox!

Step back and have a think at this stage. Already - having removed the factory snorkel - we've more than halved the Pintara's overall intake restriction. We could simply put the airbox back together and enjoy a noticeable improvement in performance, but - really - it's best to take the next step and ensure intake air temps are kept as low as possible...

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In contrast to the intake systems we've devised in the past (which used cold air induction pipes), we opted to install a 1.6mm thick galvanised partition to separate the airbox mouth from under-bonnet heat. The shield we fabricated for the Pintara is certainly nothing elaborate and took about an hour of work.

In the case of the Pintara, it was best to curve our new heat shield around the battery between the airbox and the radiator support panel. With the airbox separated from the underbonnet heat it must, of course, draw air from somewhere; it just happens the Pintara has a hole through the inner guard that is amply large in diameter (about the same cross-section as the airbox's breath hole). Again, if you're not so lucky, you'll need to create a large hole through the inner guard - this is best done with a hole-saw and file.

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The first step in the fabrication of our heat shield was to cut our sheet of galv to the appropriate size and bend it as required; the shield must be a fairly tight fit against the bodywork, otherwise there'll be gaps for under-bonnet heat to make its way to the airbox. Note that the top edge of the shield should also seal against the underside of the bonnet when it's closed.

None of this need be absolutely perfect, though, because we'll later be able to seal up any small gaps with some rubber strips - as you'll see...

With the shield fitting into position satisfactorily, we then whipped up a couple of heat shield mounting brackets from off-cuts of steel strip. These don't have to particularly heavy-duty, since the shield doesn't bear any loads. We used a battery tray bolt as one mounting point and drilled a small hole through the inner guard for the rear shield mount.

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To ensure a good seal from under-bonnet heat, we used length of U-shape rubber strip along each edge of the shield; these helped fill any gaps against the bodywork and the (closed) bonnet. Note that the rubber strip can be curved around gentle radius bends, but for any sharp 90-degree bends you should cut the rubber strip at opposing 45-degree angles (as seen here). This looks neater than a plain ol' 90-degree butt joint. A line of superglue was used to permanently connect the rubber strips to the shield.

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Next, we hit our new shield with some black spray paint and guess what? Once we put the intake back together, reattached the relay box, overflow bottle and guard liner we were finished!

Results...

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With the Pintara's new air intake polished off in just a couple of hours (including fumbling about taking photographs) we returned to the road to grab some 'after' flow restriction and intake temperature measurements. The result? How about zero restriction prior to the airbox (bringing total intake restriction down to just 5.5 inches of water) and intake temperatures about 30-degrees Celsius (down from about 45-degrees Celsius)?!

On-road performance was also improved, but - given we'd removed a relatively modest 6.5 inches of water flow restriction - we're talking only a small amount. Nought to 100 km/h sprints now take 9.2-seconds, bettering our previous best by about a tenth. More noticeable than a top-end power gain, however, is the improvement in throttle response that can be felt in almost all circumstances. The only downside of the modification is increased induction noise - some people would call this a "sexy howl" but it doesn't do much for us....

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At the end of this little spurt of effort, though - and given the total cost of materials was under $20 - we're quite happy with the overall improvement.

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